106 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
reef, but also a barrier reef of a triangular shape surrounding it. It is 
evident that this barrier has been formed upon the denuded and eroded 
spurs of Totoya, which once extended seaward from the outer rim of the 
volcano. 
Supposing, now, that the erosion of both Thombia and Totoya had 
continued for a period of time long enough to have reduced the rims of 
these volcanoes to the level of the sea, we should have, as soon as corals 
had covered the flats thus formed indicating the former existence of the 
rim, atolls of nearly circular form; the one with a circumference of only 
two miles, and a depth of 24 fathoms, without patches in the central 
lagoon; the other munch larger, more than 25 miles in circumference, 
having a depth of 34 fathoms inside the lagoon, which would be dotted 
with patches, some of them forming part of the rim, others being the 
remains of eroded spurs extending towards the centre of the extinct 
crater. 
Admiral Wharton? has given a most interesting sketch of Clipperton 
Atoll, in which he confirms the trachytic nature of the “ Rock ” of Clip- 
CLIPPERTON ROCK, 
(From a Photograph by J. T. Arundel.) 
perton, as determined by Professor Wolff? from specimens collected by 
Mr. Jensen and kindly sent me by Professor Davidson. The photo- 
graphs and specimens collected by Mr, J. T. Arundel, on which Admiral 
Wharton’s notice is based, have enabled him to give what seems to me 
a natural explanation of the character of the atoll. As he says, it is 
1 Quart. Journ. Geol. Soc. of London, May, 1898, p. 228. 
2 Bull. Mus. Comp. Zodl., Vol. XX VI. No. 1, p. 174, 1894. 
